Dwarf Galaxy Holmberg II in Ursa Major
February 2022 - Picture of the Month
Back to galaxies for February and a member of the M81 group in Ursa Major that lies only about 10 million light-years from us. Holmberg II is a dwarf irregular galaxy, and I don't think it's too hard to see why, on the edge of its galaxy group safely away from the gravitational effects of the larger members. All the same, it is forming plenty of young massive blue stars which can be seen in David's image as the blue regions, giving the galaxy a knotty appearance.
This galaxy's low mass and quiet neighbourhood allow structures created by generations of stars to survive for a relative long time, giving the Hubble Space Telecope (HST) a chance to image them. This it duly did, in a spread of wavelengths from the visible to near-infrared, revealing huge bubbles in its interstellar medium created by a succcession of solar winds of massive stars and supernovae. Sequential star formation has inflated these bubbles to 300–5,500 light-years in diameter.
Holmberg II is also home to an ultraluminous X-ray source, the location of which David has indicated in a crop of this image on his website. The source itself is at the intersection of three gas bubbles. This may be an intermediate mass black hole that's drawing on the abundant gas around it.
There are some reports of observing Holmberg II visually, but it would need a large aperture telescope and dark sky.
James Whinfrey - Website Administrator.
